The present invention relates to magnetic deflection of an electron beam in the cathode ray tube (CRT) of a video display and, more particularly, to means for minimizing the residual inductances in a ferromagnetic deflection coil system.
An electron beam in the CRT of a video display is deflected by means of multiple deflection coils as it scans the face of the CRT. It is by means of magnetic fields generated by the current-carrying deflection coils that the electron beam is accurately positioned on the CRT's faceplate. These deflection coils are generally mounted in the neck of the CRT with a sawtooth current producing the magnetic field required for rapid scanning and retrace by the electron beam. During the retrace period in which the beam is moved in a direction opposite to its scanning direction, the current in the deflection coils is reversed. However, due to self-resonances in the deflection coil drive circuit the deflection field does not entirely collapse. This residual energy and resulting magnetic field is evidenced by scan distortion following the retrace pulse. This is evident to the viewer as distortion or discontinuities on the extreme left-hand portion of the video display during the initial scanning phase of individual horizontal lines. The scanning electron beam undergoes velocity modulation at the start of trace on the extreme left side of the CRT faceplate due to the resonances present in the inductances of the deflection coils.
In a typical video display there are three sources of magnetic fields sufficient in magnitude to cause the aforementioned video presentation distortions. These three sources are the linearity coil, or lin coil, the horizontal width coil and the horizontal yoke deflection system. The lin coil generally is an inductor biased with a magnet so that its inductance changes with the amount and polarity of current passing through it. To compensate for internal losses due to the voltage drop across the lin coil, the width coil and the deflection yoke, the lin coil provides the deflection yoke with a distorted sawtooth voltage waveform during scan which compensates for the nonlinearity inherent in the deflection system. The horizontal width coil generally is a variable inductor capable of varying the amount of voltage provided to the deflection yoke. By thus varying the current passing through the yoke the width of the video display's presentation may be varied. The deflection yoke is the third, and most important, source of self-resonance resulting in scanning distortions following retrace. The yoke is comprised of the horizontal and vertical deflection coils for precisely positioning the electron beam on the CRT's fluorescent screen. Current of varying magnitude passed through the yoke generates the magnetic fields which control the deflection of the electron beam. It is the magnetic field produced by the self-resonance present after the termination of current through these components which causes distortion in the horizontal scan. This problem, while not significant in a television receiver which is overscanned, i.e., a receiver in which the horizontal beam scansion extends beyond the edges of the CRT, is particularly troublesome in a video display used in a non-television application since such a display is significantly underscanned.
The general approach disclosed in the prior art for solving this problem involves resistive loading of the lin and width coils and the yoke at all times during circuit operation. However, this attempt to filter out these spurious yoke signals results in unacceptably high power consumption. This is particularly undesirable in the case of a general application video display which typically is driven by a system of low power output.
One example of this approach is described in U.S. Pat. No. 3,456,149. Therein is described a means for reducing the residual magnetism in the ferromagnetic yoke of a CRT deflection system involving the incorporation of separate two-terminal networks across each of the deflection coils. Each two-terminal network is comprised of an adjustable resistor and a compensating coil with each network arranged to partially bridge in a shunt relationship its associated deflection coil.
Another approach to solving this problem is disclosed in U.S. Pat. No. 3,502,939 wherein is described a distortion-correcting apparatus for a magnetic deflection device including a magnetic core in the form of a cubic Wheatstone bridge configuration, a pair of coils respectively wound about the diagonal magnetic members of the Wheatstone bridge and means to connect the coils to an orthogonal magnetic field generating circuit with at least one of the coils being a deflection current generating coil. The deflection current generating coil acts as an output winding of the magnetic deflection device's flyback transformer.
The first approach described above is of a relatively simple design in which the adjustable resistor-compensating coil network acts to reverse the direction of current through the deflecting coil thus canceling out the residual magnetic field produced by that coil. The compensating coil in this network, however, produces its own magnetic field which must be taken into account in designing the CRT's deflection system. The second approach described above involves the generation of a compensating magnetic field. It represents a considerably more complex solution to this problem when such additional considerations as leakage fields and eddy currents in metal components must be addressed. In addition, the components of the prior art system must be precisely positioned in the video display relative to the existing magnetic deflection system.
The present invention, on the other hand, offers a more simple, efficient and effective solution to eliminating the residual inductance and resulting magnetic field of the electron beam deflection system. The present invention also represents a more direct and less complicated approach than the second invention discussed above.